the stem is shaped so the planking runs smoothly across the side of the stem. It makes a constantly changing angle on the side of the stem which gradually changes to one constant bevel. It allows the planking to come together at the top of the stem.
Its kind of hard to explain but hopefully the pictures will make it clearer.
The object is that you don't want the planking to just be resting in the corner of the stem but to shape a flat space for the planking to rest against and to be nailed to.